1. Field of the Invention
The invention relates to electric motors and more particularly to a permanent magnet-excited small motor.
2. Description of the Prior Art
A permanent magnet-excited small motor of the type with which this invention is concerned is known from German Utility Model DE 84 28 487. The permanent magnet-excited small motor described in this reference is used for instance as a drive mechanism for a motor vehicle window control. It has a rotor, rotatably supported about a pivot axis, that includes a commutator and an armature put together from a lamination package. Two permanent magnet poles surrounding the armature at opposed portions are also provided, which communicate with one another via a pole housing that encloses the armature and the permanent magnet poles annularly, so that the magnetic short circuit is carried via the pole housing. The pole housing is injection-molded into a plastic motor housing. In the regions between the permanent magnet poles, both the pole housing and the plastic motor housing surrounding the pole housing each have flat faces, so that the pole housing and the motor housing are embodied in a space-saving way and require only a small installation space, for instance in a door of the vehicle.
A disadvantage of the known permanent magnet-excited small motor, however, is that noise emissions in the flattened pole housing are increased, compared to a completely round structural design. Moreover, the motor performance and the torque generated by the small motor are reduced because of the transverse armature field between the flattened side regions.
The permanent magnet-excited small motor of the present has the advantage over the prior art that noise emissions are reduced markedly, compared with the permanent magnet-excited small motor with completely plane flat faces of the pole housing as known from the prior art. Since according to the invention the flat faces of the pole housing are curved convexly outward in at least some portions, a certain rounding of the pole housing is attained despite the flat faces provided. On the one hand, this reduces the impetus to vibration in the region of the flat faces of the pole housing, since a greater spacing is established between the pole housing and the armature, and the vibration-inducing forces are thus reduced. On the other, a reduced propagation of structure-borne sound of the pole housing is also obtained, because of a diffusor effect engendered by the rounding.
At the same time, because of the increased spacing between the armature and the flat faces of the pole housing, the cross induction, which extends perpendicular to the axis connected the two permanent magnet poles and connects the two flat faces to one another via the armature, is reduced. The reduction in the cross induction leads to an increased torque of the permanent magnet-excited small motor and thus to a higher motor performance.
Despite the convexly outward-oriented curvature of the flat face of the pole housing as provided by the invention, the pole housing requires only relatively little installation space, which compared with a plane embodiment of the flat faces is at most only slightly increased and compared with a completely round design of the pole housing is reduced still more markedly. The small motor of the invention can therefore be installed in a space-saving way, for instance in motor vehicles.
The flat faces can be convexly curved outward either entirely or only in some portions. In the latter case, plane regions can be provided adjacent the permanent magnet poles, and convexly outward-curved regions can be provided between the plane regions; the curved regions then advantageously have a center point of curvature that coincides with the pivot axis of the armature, so that the curved regions surround the armature at a uniform spacing.
If the convex curvature is shaped such that the radial spacing between the flat faces and the armature is markedly greater than the radial spacing between the permanent magnet poles and the armature, the cross induction is reduced markedly compared to the primary armature field.